Weeds on ridges in fields, for instance, are likely to be pest nests, and thus need to be cut several times a year. This work is hard, and thus various machines have been proposed for practical use. Of those machines, bush cutting machines are widely used because of their smallness and ease of handling.
A bush cutting machine has a drive shaft inserted through a tubular operating pole and rotated by an engine provided at one end of the operating pole to rotate a cutting blade provided at the other end of the operating pole. An operator hangs the bush cutting machine on his shoulder, grips a bar-shaped handle provided at a middle portion of the operating pole and swings the operating pole right and left and backward and forward, thereby cutting weeds by the cutting blade. Such bush cutting machines are known in various types (e.g., see Japanese Utility Model Laid-Open Publication No. HEI-1-24336).
The above-described conventional bush cutting machine will be described with reference to FIGS. 15 and 16.
A bush cutting machine 200 shown in FIG. 15 has a structure in which one end of a drive shaft 205 is connected to an output shaft 202 of an engine 201 via a magnetic wheel 203 and a centrifugal clutch 204; a cutting blade (not shown) is connected to the other end of the drive shaft 205; and the drive shaft 205 is inserted through a tubular operating pole 206.
As shown in FIG. 16, an inlet 201a is connected to an air cleaner 212 via a carburetor 211. An exhaust muffler 213 is connected to an outlet 201b. The engine 201, the magnetic wheel 203 and the centrifugal clutch 204 are covered by a cover 214. The exhaust muffler 213 is covered by a muffler cover 215. A flow of air produced by a plurality of fans 216 attached to the magnetic wheel 203 cools the outer surface of the engine 201 and the outer surface of the exhaust muffler 213, and then is discharged into the air through a plurality of exhaust vents 215a formed in the muffler cover 215. Exhaust from the engine 201 is discharged from the exhaust muffler 213, and then is discharged through the exhaust vents 215a into the air.
In recent years, further improvements in agricultural work environment have been required. As for the bush cutting machine 200, a further reduction in the exhaust noise of the engine 201 is increasingly required to reduce an operator's burden.
In view of the workability and operability of the bush cutting machine 200, it is required that the bush cutting machine 200 and the engine 201 thereof be reduced in size and weight. With reductions in size and weight of the engine 201, the exhaust muffler 213 has to be reduced in size. The small exhaust muffler 213 has a small capacity, and has its limitations in reducing exhaust noise. In addition, the engine 201 and the exhaust muffler 213 are located near an operator because of the structure of the bush cutting machine 200, and the arrangement cannot be changed. In order to reduce an operator's burden, it is necessary to make improvements in reduction of the exhaust noise from an engine in a bush cutting machine without increasing the size of the bush cutting machine.